This invention relates to improvements in a hydrostatic steering system having an unbalanced piston servomotor to which pressurized fluid, regulated by a metering pump, is fed from a steering control valve.
Hydrostatic steering systems of the foregoing type are commonly used, for example, in heavy construction vehicles such as loaders, excavators, tractors, stackers, etc. Mechanical steering rod mechanisms for such vehicles have been replaced by oil or fluid column devices for steering force transmission which may be manually operated in the absence of hydraulic support. Steering arrangements of such type are described, for example, in German patent publications No. 1,550,557 and OS No. 27,49,353.
The metering pump associated with the foregoing type of fluid steering system usually includes a rotor enclosed by an internally toothed stator in mesh with external teeth on the rotor. Fluid displacing pockets are formed between the meshing teeth profiles of the pump rotor and stator and a slide control valve is rotated in synchronism with the pump rotor to control distibution of fluid metered by the pump rotor all in response to rotational movement imparted to a hand steering wheel. The fluid is pressurized by a powered pump. Thus, fluid powered steering of dirigible vehicle wheels is synchronized with angular movement of the hand steering wheel, the wheels being turned by piston operating rods extending from fluid piston servomotors. Because of the use of such piston operating rods, each piston servomotor has differential piston area surfaces exposed to opposed pressure chambers.
In order to prevent cavitation in the fluid supply conduits extending to the piston servomotors, one-way check valves are usually built into such pressure conduits. Where the power pump for the steering system serves other fluid operated systems for example, the pressure therein may effect opening of the one-way check valves to pressurize the supply conduits in response to development of a pressure head in the return line to the fluid reservoir in the neutral position of the steering control valve. This condition may also occur because of an accumulation of excessive fluid (due to an increase in flow-through resistance). As a result of the differential piston area surfaces in the servomotor, the servomotor will be unbalanced by a pressure force exerted in the direction of the smaller piston area surface, causing an increase in pressure in associated supply conduit and closing of the check valve therein. The pressure increase caused by the foregoing condition will drive the metering pump and circulate fluid as a motor to equalize the fluid volumes in the opposed pressure chambers of the servomotor by opening of a check valve to supply fluid from the larger surface pressure chamber to the smaller surface pressure chamber. Thus, a "run-away" action of the servomotor occurs, dependent on the pressure head developed in the return line, that is felt by the vehicle driver through the hand steering wheel in the neutral position as a sudden wheel turning force inducing the driver to apply a compensating counterforce.
It is therefore an important object of the present invention to provide a hydrostatic steering system of the aforementioned type, wherein pressure heads developed in the neutral position of the steering control valve will not create unintentional forces applied to the piston servomotor.